Polyamides are conventionally prepared by the condensation polymerization of a diacid, such as adipic acid, and a diamine, such as hexamethylene diamine, or by the polymerization of lactams, such as ε-caprolactam. Other processes are known which involve preparation of polyamides by reaction of ω-aminonitriles with water. U.S. Pat. No. 2,245,129 discloses preparation of polyamides by heating ω-aminonitriles in the presence of water. U.S. Pat. No. 6,069,228 describes a method for producing polyamide polymers via pre-polymer formation.
Polyamide pre-polymers produced from ε-caprolactam or ω-aminocapronitrile contain a significant amount of caprolactam and its oligomers as impurity. As used herein the term “pre-polymer” refers to polymeric material that is used to produce Nylon-6. The amount of caprolactam and its oligomers must be reduced in the pre-polymer in order to make a polymer of adequate quality for commercial applications. It has been reported that volatile impurities content might be reduced by evaporation at sub-atmospheric pressure and at elevated temperature (see WO 2001074925 A1, EP 989150 A1).
Nylon-6 pre-polymer can be obtained by heating caprolactam and water. When this process is employed the Nylon-6 pre-polymer typically contains about 8% (wt) caprolactam, and about 1% (wt) caprolactam dimer. It is commercially desirable for the nylon-6 pre-polymer to contain less than 1% (wt) of caprolactam and less than 0.1% cyclic dimer of caprolactam.
A commercially practiced process to reduce the content of caprolactam and its oligomers in the nylon 6 pre-polymer involves extraction by water. The technique effectively reduces the content of caprolactam and its oligomers, however, it requires a long residence time. The typical commercial water extraction step takes about 24 hours and uses hot water (about 100° C.).
For cost reasons, it would be advantageous for a commercial process to reduce the residence time of the water extraction step and/or use lower temperature water.
The present invention provides for a process that involves a relatively short duration water extraction to achieve the desired purity of nylon 6 pre-polymer.
U.S. Pat. No. 6,525,167 discloses a process for producing polyamides having a reduced extractables content. The process described therein employs a first pressure drop of at least 10 bar, and a second pressure drop of at least 20 bar. Among other things, the referenced patent does not disclose how to obtain low molecular impurity in Nylon pre-polymer achieved by a smaller pressure drop followed by water extraction.